1. Field of the Invention
The present invention relates to a technique of printing dots on the surface of a printing medium with a dot print head.
2. Description of the Related Art
Serial scan-type printers and drum scan-type printers are dot recording devices which record dots with a print head while carrying out scans both in a main scanning direction and a sub-scanning direction. There is a technique called "interlace scheme", which is taught by U.S. Pat. No. 4,198,642 and Japanese Patent Laid-Open Gazette No. 53-2040, for improving the image quality of printers of this type, especially ink jet printers.
FIG. 18 is a diagram for explaining an example of the interlace scheme. In this specification, the following parameters are used to define a printing scheme.
N: Number of nozzles; PA1 k: Nozzle pitch [dots]; PA1 s: Number of scan repeats; PA1 D: Nozzle density [nozzles/inch]; PA1 L: Sub-scanning amount [dots] or [inch]; PA1 w: Dot pitch [inch].
The number of nozzles N is the number of nozzles actually used to form dots. In the example of FIG. 18, N=3. The nozzle pitch k is the interval between the centers of the recording head nozzles expressed in units of the recorded image pitch (dot pitch w). In the example of FIG. 18, k=2. The number of scan repeats s is the number of main scans in which all dot positions on a main scanning line are serviced. In the example of FIG. 18, s=1, i.e., all dot positions on a main scanning line are serviced in a single main scan. When s is 2 or greater, the dots are formed intermittently in the main scanning direction. This will be explained in detail later. The nozzle density D (nozzle/inch) is the number of nozzles per inch in the nozzle array of the print head. The sub-scanning amount L (inch) is the distance moved in 1 sub-scan. The dot pitch w (inch) is the pitch of the dots in the recorded image. In general, it holds that w=1/(D.multidot.k), k=1/(D.multidot.w).
The circles containing two-digit numerals in FIG. 18 indicate dot recording positions. As indicated in the legend, the numeral on the left in each circle indicates the nozzle number and the numeral on the right indicates the recording order (the number of the main scan in which it was recorded).
The interlace scheme shown in FIG. 18 is characterized by the configuration of the nozzle array of the recording head and the sub-scanning method. Specifically, in the interlace scheme, the nozzle pitch k indicating the interval between the centers of adjacent nozzles is defined as an integer at least 2, while the number of nozzles N and the nozzle pitch k are selected as integers which are relatively prime. Two integers are "relatively prime" when they do not have a common divisor other than 1. Further, sub-scanning pitch L is set at a constant value given by N/(D.multidot.k).
The interlace scheme makes irregularities in nozzle pitch and ink jetting feature to thin out over the recorded image. Because of this, it improves image quality by mitigating the effect of any irregularity that may be present in the nozzle pitch, the jetting feature and the like.
The "overlap scheme", also known as the "multi-scan scheme", taught for example by Japanese Patent Laid-Open Gazette No. 3-207665 and Japanese Patent Publication Gazette No. 4-19030 is another technique used to improve image quality in color ink jet printers.
FIG. 19 is a diagram for explaining an example of the overlap scheme. In the overlap scheme, 8 nozzles are divided into 2 nozzle sets. The first nozzle set is made up of 4 nozzles having even nozzle numbers (left numeral in each circle) and the second nozzle set is made up of 4 nozzles having odd nozzle numbers. In each main scan, the nozzle sets are each intermittently driven to form dots in the main scanning direction once every (s) dots. Since s=2 in the example of FIG. 19, a dot is formed at every second dot position. The timing of the driving of the nozzle sets is controlled so that the each nozzle set forms dots at different positions from the other in the main scanning direction. In other words, as shown in FIG. 19, the recording positions of the nozzles of the first nozzle set (nozzles number 8, 6, 4, 2) and those of the nozzles of the second nozzle set (nozzles number 7, 5, 3, 1) are offset from each other by 1 dot in the main scanning direction. This kind of scanning is conducted multiple times with the nozzle driving times being offset between the nozzle sets during each main scan to form all dots on the main scanning lines.
In the overlap scheme, the nozzle pick k is set at an integer at least 2, as in the interlace scheme. However, the number of nozzles N and the nozzle pitch k are not relatively prime, but the nozzle pitch k and the value N/s, which is obtained by dividing the number of nozzles N by the number of scan repeats s, are set at relatively prime integers instead.
In the overlap scheme, the dots of each main scanning line are not all recorded by the same nozzle but by multiple nozzles. Even when the nozzle characteristics (pitch, jetting feature etc.) are not completely uniform, therefore, enhanced image quality can be obtained because the characteristics of the individual nozzles is prevented from affecting the entire main scanning line.
As described above, a variety of dot recording schemes have been proposed so far. These conventional dot recording schemes, however, have a problem that the dots recorded by a certain pass of main scan are often adjacent to the dots recorded by a previous pass immediately before the certain pass. In the specification hereof, the expression "two dots are adjacent to each other" includes not only the case where one dot is present at the position immediately above, immediately below, on the immediate left of, or on the immediate right of another dot, but the case where one dot is present at the position oblique to another dot. Namely a certain dot is "adjacent" to another dot when the certain dot is present at any one of eight neighbor positions to another dot.
Recording a new dot in a current pass at the position adjacent to another dot recorded in the previous pass often causes ink blot or deviation of dot density. The conventional dot recording schemes thus result in undesirably deteriorating image quality.